When the power goes out, the sump will fill up with any water that's left in the main tank above the level of the overflow/s. If you have too much water in the sump, the sump will overflow. So the trick is to leave enough space in the sump to compensate for the extra water during a power outage. This will vary tank by tank. Sometimes you'll only need to leave a few inches of space at the top. Sometimes you'll need to leave 6 or more. You'll need to do a few "practice" runs where you turn off the return pump and let the main tank drain into the sump. Bail water from the sump if the water level gets too high. When you get to the point where the main tank stops draining into it, and the sump is not overflowing, you know that this is the safe level where there's enough space in the sump to hold any extra water during a power outage. When you turn the return pump back on, keep the water at this level, and mark it with a sharpie so that you know not to go above that water level.
You will also need to have a one way valve in your return line or small holes drilled in your return line output to prevent a back siphon from draining your main tank in a power outage.